I forward, here, a message from Dr. Jaime Willbur, Potato and Sugar Beet Extension Pathologist with Michigan State Univ., regarding the late blight situation in Michigan. The genotype or clonal lineage of the late blight pathogen is US-23. This has been the predominant type in North America in recent years. While some metalaxyl/mefenoxam resistance can be found in the pathogen population members in the US-23 type, it is generally quite sensitive to the fungicide. My lab has worked on US-23 Phytophthora infestans isolates over the past decade and we’ve detected some insensitivity, but most isolates have been sensitive – meaning that you can control the late blight with metalaxyl and/or mefenoxam fungicides.
Potato foliage should continue to be protected with fungicides active against late blight. During Sep 10-12, we had a significant accumulation of Disease Severity Values which indicate weather conditions highly promotive to late blight onset. Additionally, it’s important to remember that because airborne sporangia can be deposited on soil and move downward with water to cause tuber infections, it is important to consider protection of the crop even after vine senescence in some areas at highest risk. I’m not aware of any late blight on potato or tomato in Wisconsin at this time, but any field with active sporulation on foliage can certainly be a risk to tubers in a senesced field. Late season mancozeb treatments can aid in limiting the success of tuber infections.
Our UW Plant Disease Diagnostic Clinic and my research laboratory are always available to assist in diagnosing suspect late blight at no charge. My lab will also advance positive samples to clonal lineage/strain typing.